Beta titanium alloy

ABSTRACT

A beta titanium alloy comprises 25 wt % vanadium, 15 wt % chromium, 2 wt % aluminium, up to 0.15 wt % oxygen, 0.1 to 0.3 wt % carbon and the balance titanium plus incidental impurities. The carbon is present in the form of titanium carbide precipitates distributed throughout the beta titanium alloy matrix, the titanium carbide precipitates refine the grain size of the beta titanium alloy matrix and remove oxygen from the beta titanium alloy matrix to reduce precipitation of alpha titanium in the beta titanium alloy matrix to increase the ductility of the beta titanium alloy. The alloy is useful for gas turbine engine compressor blades ( 10 ), compressor vanes, compressor casings etc.

[0001] The present invention relates to beta titanium alloys,particularly to burn resistant beta titanium alloys.

[0002] Titanium alloys are used in gas turbine engines, particularly forcompressor blades and compressor vanes in the low pressure compressorand the high pressure compressor.

[0003] A problem associated with titanium alloys is that titanium is ahighly reactive metal and may burn in appropriate circumstances. Forexample if the tip of a titanium alloy compressor blade rubs on thecompressor casing, during operation of the gas turbine engine, thefriction may lead to ignition of the titanium alloy compressor blade.

[0004] Thus there is a requirement for a titanium alloy which is burnresistant, preferably it does not burn, if friction occurs between atitanium alloy compressor blade and a compressor casing, duringoperation of the gas turbine engine.

[0005] Additionally there is a requirement for such a titanium alloy tobe ductile and there is a requirement for such a titanium alloy to berelatively cheap in terms of raw products and processing requirements.

[0006] A non burning beta titanium alloy is known from published UKpatent application GB2238057A, which comprises at least 20 wt %vanadium, at least 10 wt % chromium and at least 40 wt % titanium. Thisalloy may comprise up to 2.5 wt % carbon and up to 0.3 wt % oxygen. Thismentions that the carbon addition improves the post creep ductility ofthe alloy and the carbon forms carbides. There is no discussion of theoxygen in the alloy. None of the alloy examples comprise oxygen. Thealloy does not comprise any aluminium. Thus this alloy is relativelyexpensive to produce because the vanadium is added as an element ratherthan as a vanadium-aluminium master alloy.

[0007] A beta titanium alloy is known, from UK patent GB1175683 which,comprises 25-40 wt % vanadium, 5-15 wt % chromium, 0-10 wt % aluminiumand the balance titanium and impurities. This alloy may comprise up to 2wt % carbon and up to 0.3 wt % oxygen. The carbon is added to increasethe strength of the alloy and the oxygen is an impurity. None of thealloy examples comprise oxygen. This alloy is relatively cheap toproduce because the vanadium is added in the form of vanadium-aluminiummaster alloy.

[0008] Accordingly the present invention seeks to provide a novel betatitanium alloy which minimises the above mentioned problem.

[0009] Accordingly the present invention provides beta titanium alloycomprising at least 10 wt % of one or more beta stabilising elements,0.1 to 0.4 wt % carbon up to 0.2 wt % oxygen and the balance titaniumand incidental impurities, wherein the carbon is present in the form oftitanium carbide precipitates distributed throughout the beta titaniumalloy matrix, the titanium carbide precipitates refine the grain size ofthe beta titanium alloy matrix and remove oxygen from the beta titaniumalloy matrix to reduce precipitation of alpha titanium in the betatitanium alloy matrix to increase the ductility of the beta titaniumalloy.

[0010] Preferably the beta stabilising element are selected from thegroup comprising vanadium, molybdenum, tantalum, niobium, chromium,tungsten, manganese and iron.

[0011] Preferably the beta titanium alloy comprises aluminium.

[0012] Preferably the present invention provides a beta titanium alloycomprising 20 to 30 wt % vanadium, 13 to 17 wt % chromium, 1.0 to 3.0 wt% aluminium, 0.1 to 0.4 wt % carbon, up to 0.2 wt % oxygen and thebalance titanium plus incidental impurities.

[0013] Preferably the beta titanium alloy comprises 1.5 to 2.5 wt %aluminium.

[0014] Preferably the beta titanium alloy comprises 0.15 to 0.3 wt %carbon.

[0015] Preferably the beta titanium alloy comprises less than 0.15 wt %oxygen.

[0016] Preferably the beta titanium alloy comprises 23-27 wt % vanadium,13-17 wt % chromium, 1-3 wt % aluminium, up to 0.15 wt % oxygen, 0.1 to0.3 wt % carbon and the balance titanium plus incidental impurities.

[0017] Preferably the beta titanium alloy comprises 25 wt % vanadium, 15wt % chromium, 2 wt % aluminium, up to 0.15 wt % oxygen, 0.1 to 0.3 wt %carbon and the balance titanium plus incidental impurities.

[0018] The present invention also provides an article comprising a betatitanium alloy, the beta titanium alloy comprising at least 10 wt % ofone or more beta stabilising elements, 0.1 to 0.4 wt % carbon up to 0.2wt % oxygen and the balance titanium and incidental impurities, whereinthe carbon is present in the form of titanium carbide precipitatesdistributed throughout the beta titanium alloy matrix, the titaniumcarbide precipitates refine the grain size of the beta titanium alloymatrix and remove oxygen from the beta titanium alloy matrix to reduceprecipitation of alpha titanium in the beta titanium alloy matrix toincrease the ductility of the beta titanium alloy.

[0019] Preferably the beta titanium alloy comprises 20 to 30 wt %vanadium, 13 to 17 wt % chromium, 1.0 to 3.0 wt % aluminium, 0.1 to 0.4wt % carbon, up to 0.2 wt % oxygen and the balance titanium plusincidental impurities. Preferably the beta titanium alloy comprises23-27 wt % vanadium, 13-17 wt % chromium, 1-3 wt % aluminium, up to 0.15wt % oxygen, 0.1 to 0.3 wt % carbon and the balance titanium plusincidental impurities.

[0020] Preferably the beta titanium alloy comprises 25 wt % vanadium, 15wt % chromium, 2 wt % aluminium, up to 0.15 wt % oxygen, 0.1 to 0.3 wt %carbon and the balance titanium plus incidental impurities.

[0021] Preferably the article comprises a component for a gas turbineengine.

[0022] Preferably the component comprises a compressor blade or acompressor vane.

[0023] The component may comprise a tip portion for a compressor blade.

[0024] The present invention will be more fully described by way ofexample with reference to the accompanying drawings in which:

[0025]FIG. 1 shows a compressor blade comprising a beta titanium alloyaccording to the present invention.

[0026]FIG. 2 shows a compressor blade having a tip portion comprising abeta titanium alloy according to the present invention.

[0027]FIG. 3 is a graph of elongation against oxygen content for betatitanium alloys with varying degrees of carbon addition.

[0028] A gas turbine engine compressor blade 10, as shown in FIG. 1,comprises an aerofoil 12, a platform 14 and a root 16. The compressorblade 10 comprises a beta titanium alloy, preferably a burn resistantbeta titanium alloy, according to the present invention. The betatitanium alloy compressor blade may be forged, or cast, or produced byother thermomechanical processes.

[0029] A gas turbine engine compressor blade 20, as shown in FIG. 2,comprises an aerofoil 22, a platform 24 and a root 26. The compressorblade 10 also comprises a tip portion 28 on the extremity of theaerofoil 22 remote from the platform 24 and root 26. The tip portion 28comprises a beta titanium alloy, preferably a burn resistant titaniumalloy according to he present invention. The tip portion 28 may compriseweld filler deposited onto the aerofoil 22 by using the burn resistantbeta titanium alloy as the weld filler during welding, e.g. tungsteninert gas (TIG) welding. The weld filler subsequently being machined tosize and shape. Alternatively the tip portion 28 may comprise a block ofthe burn resistant beta titanium alloy which is welded onto theaerofoil, e.g. tungsten inert gas (TIG) welding, laser welding, electronbeam welding etc. The block subsequently being machined to size andshape.

[0030] The burn resistant titanium alloy according to the presentinvention comprises 20 to 30 wt % vanadium, 13 to 17 w% chromium, 1.0 to3.0 wt % aluminium, 0.1 to 0.4 wt % carbon, up to 0.2 wt % oxygen andthe balance titanium plus incidental impurities. Preferably the betatitanium alloy comprises 23-27 wt % vanadium, 13-17 wt % chromium, 1-3wt % aluminium, up to 0.15 wt % oxygen, 0.1 to 0.3 wt % carbon and thebalance titanium plus incidental impurities. Preferably the betatitanium alloy comprises 25 wt % vanadium, 15 wt % chromium, 2 wt %aluminium, up to 0.15 wt % oxygen, 0.1 to 0.3 wt % carbon and thebalance titanium plus incidental impurities.

[0031] The burn resistant beta titanium alloy in particular has afavourable combination of carbon and oxygen which enhances the ductilityof the burn resistant titanium alloy. It has been found that there is asynergy between the oxygen and carbon levels. In particular it has beenfound that the carbon reacts with the titanium to form titanium carbide(Ti₂C) precipitates which refine the grain size of the beta titaniumalloy matrix.

[0032] Furthermore the titanium carbide (Ti₂C) precipitates have a.affinity for the oxygen and the oxygen becomes attached to the titaniumcarbide (Ti₂C) precipitates and thus the oxygen is removed from the betatitanium alloy matrix. The presence of oxygen in the beta titanium alloymatrix has the effect of promoting the precipitation of alpha titaniumin the beta titanium alloy matrix. The presence of alpha titanium in thebeta titanium alloy reduces the ductility of the beta titanium alloy.Thus because the titanium carbide precipitates remove oxygen from thebeta titanium alloy matrix there is less oxygen available to promote theprecipitation of the alpha titanium, and thus the precipitation of alphatitanium in the beta titanium alloy matrix is reduced. Therefore thisincreases the ductility of the beta titanium alloy. It is to be notedthat the carbon does not remove all the oxygen from the beta titaniumalloy matrix.

[0033] It has been found that titanium carbide (Ti₂C) precipitates areformed when more than 0.1 wt % carbon is present in the beta titaniumalloys mentioned above. These titanium carbide precipitates getter theoxygen and refine the grains. The carbon addition improves the stabilityof the beta titanium alloys.

[0034] The increase in the ductility of the beta titanium alloy providedby the synergy between the oxygen and the carbon enables aluminium to beadded to the beta titanium alloy, and this enables the use of cheapermaster alloys, e.g. vanadium aluminium master alloys.

EXAMPLES

[0035] Alloys with the composition listed in table 1 were produced usinga plasma melter from mixtures of master alloys and elemental rawmaterials. Either titanium sponge with 0.04 wt % oxygen or titaniumgranules with 0.086 wt % oxygen were used according to the desiredoxygen levels. The base level of carbon with no deliberate addition ofcarbon is 0.02 wt % carbon which was brought in by impurities in the rawmaterials. TABLE 1 (Composition in weight %) Alloy Elements Code Ti V CrAl C O A8 Balance 25 15 2 0.02 0.065 A14 Balance 25 15 2 0.02 0.095 A12Balance 25 15 2 0.02 0.135 A17 Balance 25 15 2 0.10 0.115 A18 Balance 2515 2 0.20 0.110 A11 Balance 25 15 2 0.30 0.095 A13 Balance 25 15 2 0.090.165 A19 Balance 25 15 2 0.21 0.15 A20 Balance 25 15 2 0.31 0.15

[0036] The alloy samples were all forged at 1050° C. to produce pancakesabout 16 mm thick. The samples were then heat treated at 850° C. for 2hours air cooled, or heat treated at 1050° C. for 0.5 hours air cooledfollowed by ageing at 700° C. for 4 hours air cooled or heat treated at1050° C. for 0.5 hours air cooled then followed by ageing at 700° C. for4 hours air cooled and then followed by heat treatment at 550° C. for500 hours air cooled.

[0037] The alloy samples were cut, polished and etched for conventionaloptical microscopy and scanning electron microscopy. Additionally X-raydiffraction, EDX and transmission electron microscopy were performed onthe alloy samples. All the alloy samples were tested in tension at roomtemperature, and the results are listed in table 2 and illustratedgraphically in FIG. 3.

[0038] Condition 1:—850° C./2 hours air cooled.

[0039] Condition 2:—1050° C./0.5 hours air cooled and 700° C./4 hoursair cooled.

[0040] Condition 3:—1050° C./0.5 hours air cooled and 700° C./4 hoursair cooled and 550° C./500 hours air cooled.

[0041] The carbon in alloys A8, A14 and A12 are impurities in the alloyrather than deliberate addition of carbon. TABLE 2 (Tensile Properties)0.2% Ultimate Proof Tensile Alloy Heat Treat Stress Strength ElongationCode Conditions (MPa) (MPa) (%) A8 1 828 858 21.0 A14 1 805 842 1.5 2892 892 1.4 3 953 955 0.6 A12 1 835 853 0.5 2 902 0.1 3 949 949 0.3 A171 916 921 24.0 2 878 891 16.4 3 887 896 4.9 A18 1 899 939 20.3 2 894 92315.0 3 849 891 11.8 A11 1 867 905 16.6 2 866 882 14.0 3 849 891 4.6 A131 891 0 2 938 944 0.5 A19 1 900 914 8.4 2 882 903 8.7 3 890 911 9.9 A201 935 964 10.9 2 903 915 1.0 3 885 927 10.5

[0042] It is clear from table 2, and FIG. 3, that when there is nodeliberate carbon addition the trend is for the elongation, theductility, to decrease with increasing oxygen levels. It is also clearthat when carbon is added the elongation, ductility, is improved. It isseen that there is a significant increase in ductility by adding over0.1 wt % carbon to beta titanium alloys with 0.095 to 0.ll5 wt % oxygen,see alloys A14, A17, A18 and All. The improvement in ductility foralloys with 0.15 to 0.165 wt % oxygen and over 0.2 wt % carbon is alsosignificant for most of the heat treatments. It is seen that theductility of the beta titanium alloys without carbon additiondeteriorates after heat treatment condition 3. This is due toprecipitation of alpha titanium in the beta titanium alloy matrix. Theductility of the beta titanium alloys with carbon addition alsodeteriorates after heat treatment. However, some ductility is retained.Also the ductility of alloys A19 and A20 with high carbon and highoxygen levels have greater ductility than alloy A17 with lower carbonand oxygen levels after heat treatment condition 3.

[0043] Examination showed that the alloy samples without carbon failedby cleavage fracture, whereas alloy samples with carbon failed by aductile, or by a mixture of ductile/brittle, manner.

[0044] The addition of carbon overcomes the detrimental effects thatoxygen and alpha titanium have on the room temperature ductility of thebeta titanium alloy and the metallurgical stability of the beta titaniumalloy after high temperature exposure.

[0045] The titanium carbide precipitates formed are stable to heattreatment and these titanium carbide precipitates refine the as forgedand heat treated microstructure and as cast microstructure. The refinedmicrostructure may deform more uniformly and may have an effect on theductility of the beta titanium alloy. The titanium carbide precipitatesgetter oxygen, increase the ductility of the beta titanium alloy matrixand suppress the formation of the alpha titanium in the beta titaniumalloy matrix. The refined beta titanium alloy matrix has smaller grainsand thus there are more grain boundaries. The amounts of alpha titaniumprecipitation present on each grain boundary is less and this furtherincreases ductility by reducing embrittlement due to alpha titanium. Thecarbon level must not be too high in the beta titanium alloys, since theprecipitation of too much titanium carbide is detrimental to ductility.

[0046] It is well known in the art that the addition of carbon to betatitanium alloys produces titanium carbides. It is also well known thatbeta titanium alloys become brittle due to titanium carbideprecipitation. Thus this improvement in ductility of the beta titaniumalloy due to the higher than normal addition of carbon in the presenceof the oxygen is completely unexpected.

[0047] Although the invention has been described with reference to anarrow range of beta titanium alloys it is believed that it isapplicable to all beta titanium alloys with more than 10 wt % of one ormore beta stabilising elements and oxygen present, which decreases theductility of the beta titanium alloy by stabilising alpha titanium, oralpha 2 titanium, in the beta titanium alloy matrix. The betastabilising element may be one or more of the elements vanadium,molybdenum, tantalum, niobium, chromium, tungsten, manganese, copper,nickel and iron.

[0048] The advantages provided by the present invention are an increasein the ductility of the beta titanium alloy provided by the synergybetween the oxygen and the carbon. This enables aluminium to be added tothe beta titanium alloy, and this enables the use of cheaper masteralloys, e.g. vanadium aluminium master alloys. There may also be animprovement in the processing temperature range.

[0049] The prior art mentioned above does not disclose, or suggest, thatthere is a synergy between carbon and oxygen in beta titanium alloyswhich increases the ductility of the beta titanium alloy.

[0050] Although the invention has been described with reference to theuse as compressor blades and compressor vanes, it may also be used tomake compressor casings and other suitable components for gas turbineengines or other engines and for other applications.

We claim:
 1. A beta titanium alloy comprising 20 to 30 wt % vanadium, 13to 17 wt % chromium, 1.0 to 3.0 wt % aluminium, 0.1 to 0.4 wt % carbon,up to 0.2 wt % oxygen and the balance titanium plus incidentalimpurities, wherein the carbon is present in the form of titaniumcarbide precipitates distributed throughout the beta titanium alloymatrix, the titanium carbide precipitates refine the grain size of thebeta titanium alloy matrix and remove oxygen from the beta titaniumalloy matrix to reduce precipitation of alpha titanium in the betatitanium alloy matrix to increase the ductility of the beta titaniumalloy.
 2. A beta titanium alloy as claimed in claim 1 wherein the betatitanium alloy comprises 1.5 to 2.5 wt % aluminium.
 3. A beta titaniumalloy as claimed in claim 1 wherein the beta titanium alloy comprises0.15 to 0.3 wt % carbon.
 4. A beta titanium alloy as claimed in claim 1wherein the beta titanium alloy comprises less than 0.15 wt % oxygen. 5.A beta titanium alloy as claimed in claim 1 wherein the beta titaniumalloy comprises 23-27 wt % vanadium, 13-17 wt % chromium, 1-3 wt %aluminium, up to 0.15 wt % oxygen, 0.1 to 0.3 wt % carbon and thebalance titanium plus incidental impurities.
 6. A beta titanium alloy asclaimed in claim 1 wherein the beta titanium alloy comprises 25 wt %vanadium, 15 wt % chromium, 2 wt % aluminium, up to 0.15 wt % oxygen,0.1 to 0.3 wt % carbon and he balance titanium plus incidentalimpurities.
 7. An article comprising a beta titanium alloy, the betatitanium alloy comprising 20 to 30 wt % vanadium, 13 to 17 wt %chromium, 1.0 to 3.0 wt % aluminium, 0.1 to 0.4 wt % carbon, up to 0.2wt % oxygen and the balance titanium plus incidental impurities, whereinthe carbon is present in the form of titanium carbide precipitatesdistributed throughout the beta titanium alloy matrix, the titaniumcarbide precipitates refine the grain size of the beta titanium alloymatrix and remove oxygen from the beta titanium alloy matrix to reduceprecipitation of alpha titanium in the beta titanium alloy matrix toincrease the ductility of the beta titanium alloy.
 8. An article asclaimed in claim 7 wherein the beta titanium alloy comprises 23-27 wt %vanadium, 13-17 wt % chromium, 1-3 wt % aluminium, up to 0.15 wt %oxygen, 0.1 to 0.3 wt % carbon and the balance titanium plus incidentalimpurities.
 9. An article as claimed in claim 8 wherein the betatitanium alloy comprises 25 wt % vanadium, 15 wt % chromium, 2 wt %aluminium, up to 0.15 wt % oxygen, 0.1 to 0.3 wt % carbon and thebalance titanium plus incidental impurities.
 10. An article as claimedin claim 7 wherein the article comprises a component for a gas turbineengine.
 11. An article as claimed in claim 10 wherein the componentcomprises a compressor blade or a compressor vane.
 12. An article asclaimed in claim 10 wherein the component comprises a tip portion for acompressor blade.
 13. A beta titanium alloy comprising at least 10 wt %of one or more beta stabilising elements, aluminium, 0.1 to 0.4 wt %carbon up to 0.2 wt % oxygen and the balance titanium and incidentalimpurities, wherein the carbon is present in the form of titaniumcarbide precipitates distributed throughout the beta titanium alloymatrix, the titanium carbide precipitates refine the grain size of thebeta titanium alloy matrix and remove oxygen from the beta titaniumalloy matrix so reduce precipitation of alpha titanium in the betatitanium alloy matrix to increase the ductility of the beta titaniumalloy.
 14. A beta titanium alloy as claimed in claim 13 wherein the betastabilising element is selected from the group comprising vanadium,molybdenum, tantalum, niobium, chromium, tungsten, manganese and iron.15. A beta titanium alloy as claimed in claim 13 wherein the betatitanium alloy comprises 1.5 to 2.5 wt % aluminium.
 16. A beta titaniumalloy as claimed in claim 13 wherein the beta titanium alloy comprises0.15 to 0.3 wt % carbon.
 17. A beta titanium alloy as claimed in claim13 wherein the beta titanium alloy comprises less than 0.15 wt % oxygen.